Astronauts have time on their hands as they await word from mission control on whether they will have to attempt a space walk to make repairs with a potentially glitchy spacesuit.

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Yesterday one of two external cooling loops on board the International Space Station failed, leading ground controllers to also shut down "non-critical systems" in the Harmony node and in the Kibo and Columbus laboratories to avoid overheating. The remaining loop (loop B) can keep the station from dangerously overheating, but it can't keep those noncritical systems running, including experiments that would normally keep astronauts busy.

This morning, space station mission integration and operations manager Kenny Todd appeared on NASA TV to provide more detail on the nature of the problem and what could be done about it. "What we're having here is a failure to be able to control the temperature of the ammonia," he said, referring to the ammonia that both loops use as coolant.

The problem has been traced to a flow control valve that is supposed to mix cold ammonia coming from the space station's radiators with warmer coolant. Without proper mixing, the coolant heading back to the station is too cold, which could cause water elsewhere in the system to ice up and create yet more problems. The pump module that powers the cooling loop is outside the station. The only way to service it is for someone on board the station to suit up and pull out the misbehaving pump module, malfunctioning flow control valve, and pump and replace the assembly with a spare model that is also stored outside the station.

Space station astronauts had to do just that in 2010, when the actual pump on the same cooling loop (loop A) failed. But this time around there's a complication: Since space station astronaut Luca Parmitano endured a harrowing near-drowning experience because of a water leak in his helmet last summer, space walks have been suspended pending the arrival of a replacement suit, to be sent up to ISS on the next SpaceX Dragon cargo mission, which is slated for February. However, mission rules dictate that no arrivals take place until both coolant loops are back in operation, Todd said. The upshot: If the astronauts can't get the valve back online, an EVA (or extravehicular activity—NASA-speak for a space walk) might have to go ahead even without the replacement suit.

"There are going to be some things that we'll have to do a little bit differently in preparing for an EVA going forward," Todd said. "Just ensuring that the suit's in good shape, that we put every mitigation possible in place—and there are a few since that timeframe—to ensure that we've got the crew member protected. I think we're in good shape moving forward if we have to go do an EVA."

At the moment, ground controllers are turning the valve on and off to try to figure out what might be wrong—without actually being able to look at it. "The team is trying to manipulate this valve and trying to draw some conclusions just based on secondary cues—what the temperature is doing, what the flow rate's doing—as they move this valve," Todd said.

At the same time, they're deferring—at least until Monday—the decision on whether or not to clear next week's planned launch of the first Orbital Sciences Cygnus cargo mission.